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POPULAR SCIENCE REVIEW. 
lowest beds would be reached before even the first ten thousand 
feet were sunk to. The lines even in a carefully prepared 
section cannot be continued far beyond what is known by ob- 
servation without great risk of error. 
It will be evident that there is a great difference between the 
two kinds of geological sections we have been considering. One 
kind is deduced from minute observation and measurement; 
the other is the expression of generalisations from these facts. 
The one is positive and belongs to the region of recorded facts ; 
the other is a diagram useful for illustration. This funda- 
mental difference must never be lost sight of by the student, 
and he should be prepared to make use of the one as well as 
the other. 
Among the modes in which the second kind of geological 
sections are employed, may be mentioned their use in illustra- 
ting and explaining theoretically views in various departments 
of geology. The construction of coral islands and the mode in 
which large deep reefs are formed is one of them. In fig. 7 
will be seen an ideal section, intended to illustrate the view of 
their construction taken by M. Darwin, and since almost 
universally accepted among naturalists and geologists. An 
island is seen in the open ocean rising out of a lagoon or large 
shallow lake of salt water, and the bottom and rim of this 
lagoon consist entirely of coral. Outside the lagoon the w^ater 
deepens with great rapidit}^, but it is found by soundings that 
the coral bottom continues for a very great depth. Now it is 
known by observation that the living coral animals of the larger 
kinds, such as build up these vast walls, do not live at great 
depths, apparently failing to find air or food sufficient for their 
purposes below a limited number of fathoms. As*the depth 
at which the large coral is found is very much greater than 
that at which it can be constructed, it is assumed that the 
island must have been continually sinking during the growth 
of the coral. Originally a large island was probably above the 
water, the land reaching to within a few fathoms of the lowest 
point of the coral. A reef grew round it, and while growing 
the island gradually sunk. The sinking must have been suf- 
ficiently slow to allow of a perpetual upward growth of the 
coral ; and thus in time those vast masses have been accumu- 
lated which are now some hundred fathoms deep, and which 
fringe the whole island like a wall. The use of the diagram- 
matic section to illustrate this view of a very curious pheno- 
menon is easily recognised. For the purpose for which it is 
needed, the diagram is as good in all respects as if perfectly 
measured to an accurate scale of equal heights and distances. 
In practical geology sections of both kinds are valuable. 
Occasionally very carefully measured sections of small parts of 
